General comments

The paper by Dewald et al. describes a summer-season field study of NO₃ reactivities measured in a temperate forest in France. Measurements were made in a small clearing close to ground and on a 40 m tower. The results are discussed in terms of diurnal variations of contributions from VOCs, NO and photolysis to the overall NO₃ reactivity, with VOC reactivities found to be significant during night and day. Based on measured and estimated concentrations of OH, O₃ and NO₃, the relative importance of these reactants for the oxidation of selected BVOCs is estimated, revealing a significant contribution of oxidation through NO₃, again during night and day, at least close to ground.

The paper is relevant, well written and structured and therefore suitable for publication in ACP. However, the work would gain relevance if information on the BVOC composition were available and if there is any missing NO₃ reactivity. In other words: how well is the k^VOC understood in this environment?

Below are some comments that may be considered in a revised version of the paper.

Specific comments

Line 22: Given the strong variability and peak values of the k^VOC at night, an arithmetic mean and a standard deviation are not useful to summarize the results. Consider using a range instead or percentiles.

Line 28: The “total loss rate” L^NO₃ is later defined (Eq. 1) as the total NO₃ reactivity, i.e. a (pseudo first-order) loss rate coefficient. Loss and production rates are usually named L and P but defined as products of concentrations and rate constants resulting in units of cm^-3s^-1 (or ppb h^-1, dependent on context). k^TOT would be a better choice (see technical comments).

Line 30: Perhaps clarify that the conclusions regarding the contribution of NO₃ to α-pinene degradation and the chemical lifetimes of BVOCs are based on measured OH and O₃ concentrations and estimated steady-state NO₃ concentrations.

Line 60: Both photolysis reactions recycle NO_x but only (R7b) recycles NO₂. Consequently, (R7b) also regenerates O_x (NO₂ + O₃) but (R7a) is a net daytime loss of O_x, another perhaps underestimated daytime effect of NO₃ chemistry.

Line 120: Was the transmittance of BVOCs through the combination of sampling lines, filter, and glass flask tested? BVOCs with low volatility and high reactivity may get lost on the way to the flow tube.

Line 132: Your correction subtracts the remaining NO reactivity from the measured reactivity to derive k^VOC. Under conditions with low k^VOC and [NO]>[O₃] the uncertainties are probably greater than the stated average 26%. Does your numerical simulation and correction procedure provide realistic, condition dependent uncertainty estimates?

Line 154: “Photolysis rates” should be named photolysis rate coefficients or photolysis frequencies consistently throughout the text.

Line 157: The paper of Meusel et al., contains no information on NO₃ absorption cross sections. I assume you used IUPAC or NASA-JPL recommendations for quantum yields and cross section which should be cited here.

Line 246: A future publication on NO₃ measurements? In Sect. 3.6 it is stated that NO₃ mixing ratios were always below the LOD which is consistent with the steady-state estimates < 0.2 ppt in Fig. 8.

Technical comments

Line 95: Specify where the wind measurements were made in the caption of the figure in the Supplement (5 m, 40 m?)

Line 137: “L^NO₃ which is the loss term” maybe better: “k^TOT which is the total reactivity”

Line 231, Eq. (1): k^TOT would fit better throughout the text with the upper index reserved for reactants, i.e. k^TOT, k^VOC, k^NO but τ_NO3, J_NO3 with the lower index reserved for the target species if necessary. Define k^NO = k₅[NO] and J_NO3 = k_7a + k_7b

Line 297 ff: Better k^VOC + k^NO

Line 320 ff: Better “k_α-pinene” than “L^α-pinene”

Line 325: Better “…the ratio of production rates k₂[NO₂][O₃] and overall loss rate coefficients k^TOT”

Figs. 4, 5, 6, 8: How was the nighttime period 03:30-19:30 determined? Local noontime is close to 12:00 UTC. The night ends before sunrise and starts before sunset (checked for July 10^th).